Due to the variety of polymers available today, products often include parts made of different polymers. These parts are often joined by welding, such as by heat, radio-frequency (RF) or ultrasonic welding. In some cases, different polymers are joined by insert molding a second polymer onto a part made of a first polymer.
For example, the hook component of a hook and loop fastener is often incorporated into a foam seat cushion, called a foam bun, by insert molding the hook component onto the foam bun with the hook elements exposed for engagement with a fabric seat covering. After insert molding, the hooks of the hook component are used to attach the seat covering to the foam bun.
In this example, it is important that there be satisfactory adhesion between the hook component and the foam bun. The hook component is generally made from nylon, and is bonded, e.g., by ultrasonic stitching, to a fabric backing of polyurethane-coated polyester to which the polyurethane foam generally adheres well during the insert molding process.